The invention concerns a method for refractometer measuring, in which a beam of rays emitted from a light source is transmitted to the interface between the optical window of a refractometer and the liquid to be measured, whereby part of the beam of rays is reflected back from the liquid and part is absorbed into the liquid and an image is produced in which the position of the boundary between a light and dark area depends on the critical angle of total reflection, the critical angle being a function of concentration of liquid, and in which the boundary between the light and dark area of the image is determined by light detectors arranged in a line.
The principle of the refractometer has been known for over a hundred years. Today refractometers are fairly common in many different fields. Examples for fields in which refractometers are used include food manufacturing industry, wood processing industry, chemical industry, and research projects in general.
The essential feature of refractometer measuring is analysis of an image produced by reflection of light. The purpose of this image analysis is to detect the position of the critical angle of total reflection, i.e. the boundary in which the light area changes to a dark area.
In previously known solutions, several different principles have been employed to determine the critical angle of total reflection. One of these principles is based on the idea that the brightness of the light area remains constant irrespective of the critical angle. Thereby the amount of light obtained from the entire image area corresponds with the proportion of light area in the total area. Many commercially available instruments employ the above-mentioned principle. However, a drawback of the principle is its sensitivity to variation in the light source, photocell and optical path.
Another known principle is a solution in which the image reflected from the optical window of the refractometer is observed by a line camera. A line camera has a plurality of light detectors, e.g. 256, integrated in a line into one and the same microcircuit. The camera displays the amounts of light measured by the detectors, one at a time. The position of the critical angle of total reflection can be determined by counting the detectors exceeding the threshold value. This kind of principle is employed e.g. in the PR-01-B, PR-01-E and PR-01-S refractometers manufactured and sold by K-Patents Oy.
The above principle operates rather well, but a drawback is that a line camera has a limited resolution capacity. The detecting element has to be large in regard to the wavelength of light, but the total length of the camera cannot in practice be increased infinitely. Another drawback is that in certain situations a single erroneous measuring point may notably distort the final result.